This invention relates to vascular repair devices, and in particular intravascular stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel or coronary artery, to maintain the patency thereof. Stents are particularly useful in the treatment of atherosclerotic stenosis in arteries and blood vessels.
Stents are generally tubular-shaped devices which function to hold open a segment of a blood vessel or other body lumen such as a coronary artery. They also are suitable for use to support and hold back a dissected arterial lining that can occlude the fluid passageway. At present, there are numerous commercial stents being marketed throughout the world. While some of these stents are flexible and have the appropriate radial rigidity needed to hold open a vessel or artery, there typically is a tradeoff between flexibility and radial strength. What has been needed and heretofore unavailable is a stent which has a high degree of flexibility so that it can be advanced through tortuous passageways and can be readily expanded, and yet have the mechanical strength to hold open the body lumen or artery into which it is implanted and provide adequate vessel wall coverage. The present invention satisfies this need. That is, the stent of the present invention has a high degree of flexibility making it possible to advance the stent easily through tortuous arteries, yet the stent has sufficient radial rigidity so that it can hold open an artery or other blood vessel, or tack up a dissected lining and provide adequate vessel wall coverage.
The present invention is directed to an intravascular stent which is highly flexible along its longitudinal axis to facilitate delivery through tortuous body lumens, but which is stiff and stable enough radially in its expanded condition to maintain the patency of a body lumen such as an artery when the stent is implanted therein.
The stent of the present invention generally includes a plurality of cylindrical rings that are interconnected to form the stent. The stent typically is mounted on a balloon catheter if it is balloon expandable or mounted on a catheter without a balloon if it is self-expanding.
Each of the cylindrical rings making up the stent have a proximal end and a distal end and a cylindrical plane defined by a cylindrical outer wall surface that extends circumferentially between the proximal end and the distal end of the cylindrical ring. The cylindrical rings are interconnected by at least one undulating link which attaches one cylindrical ring to an adjacent cylindrical ring. The undulating links are highly flexible and allow the stent to be highly flexible along its longitudinal axis. The undulating links are positioned substantially within the cylindrical plane of the outer wall surface of the cylindrical rings. The design of the highly flexible interconnecting members and their placement nested within a W-shaped member provides for uniform scaffolding and a high degree of vessel wall coverage.
The undulating links may assume various configurations but generally have an undulating or wavy shape. The undulating links include a V-shaped configuration having a concave portion and a convex portion that projects radially inward and radially outward wherein the undulating links are substantially parallel to the stent longitudinal axis. This unique design of the present invention undulating links is useful in that it helps reduce the circumference and the crimped stent diameter to a very low profile thereby enabling the delivery of the intravascular stent through a tortuous vessel.
Not only do the undulating links that interconnect the cylindrical rings provide flexibility to the stent, but the positioning of the links also enhances the flexibility by allowing uniform flexibility when the stent is bent in any direction along its longitudinal axis. Further, the cylindrical rings are configured to provide flexibility to the stent in that portions of the rings can flex or bend and tip outwardly as the stent is delivered through a tortuous vessel.
In an alternative configuration, the cylindrical rings may assume an undulating shape in order to achieve greater flexibility, in conjunction with the undulating links, along the stent longitudinal axis. This undulating shape extends radially inward and radially outward which enables the cylindrical rings to more easily deform. In addition, the undulating pattern adds to the radiopacity of the stent as it increases the mass in all areas except where the undulations run longitudinally. Further, the undulations along the stent longitudinal axis enable the packing of balloon material during crimping thus increasing stent retention. Typical crimping procedures provide a uniform and tight collapse of the stent to ensure a low profile of the diameter of the stent for placement into a smaller diameter delivery catheter.
The cylindrical rings typically are formed of a plurality of peaks and valleys, where the valleys of one cylindrical ring are circumferentially offset from the valleys of an adjacent cylindrical ring. In this configuration, at least one undulating link attaches each cylindrical ring to an adjacent cylindrical ring so that the undulating inks are positioned substantially within one of the valleys. Additionally, the undulating link attaches the valley to an adjacent peak.
While the cylindrical rings and undulating links generally are not separate structures, they have been conveniently referred to as rings and links for ease of identification. Further, the cylindrical rings can be thought of as comprising a series of U-shaped and W-shaped structures in a repeating pattern. Again, while the cylindrical rings are not divided up or segmented into U""s and W""s, the pattern of cylindrical rings resemble such configuration. The U""s and W""s promote flexibility in the stent primarily by flexing and by tipping radially outwardly as the stent is delivered through a tortuous vessel.
The undulating links are positioned so that the undulating portion is within the curved part of the W-shaped portion which generally increases the amount of vessel wall coverage. Since the undulating portion does not substantially expand (if at all) when the stent is expanded, it will continue to provide good vessel wall coverage even as the curved part of the W-shaped portion spreads apart as the stent is expanded.
The cylindrical rings of the stent are plastically deformed when expanded when the stent is made from a metal that is balloon expandable. Typically, the balloon -expandable stent is made from a stainless steel alloy or similar material.
Similarly, the cylindrical rings of the stent expand radially outwardly when the stent is formed from a superelastic alloy, such as nickel titanium (NiTi) alloys. In the case of superelastic alloys, the stent is formed in its expanded state and then constrained by a sheath on the delivery system. When the stent is in the body, the sheath is removed allowing the stent to expand to its predetermined shape.
The number and location of undulating links that interconnect adjacent cylindrical rings can be varied as the application requires. Since the undulating links typically do not expand when the cylindrical rings of the stent expand radially outwardly, the links continue to provide flexibility and to also provide a scaffolding function to assist in holding open the artery. Importantly, the addition or removal of the undulating links has very little impact on the overall longitudinal flexibility of the stent. Each undulating link is configured so that it promotes flexibility whereas prior art links actually reduce flexibility of the stent.
Because of the undulating configuration of the links, the stent has a high degree of flexibility along the stent longitudinal axis, which reduces the tendency of stent fishscaling. Stent fishscaling can occur when the stent is bent and portions of the stent project outward when the stent is in the unexpanded condition. The undulating links of the present invention reduce the likelihood of fishscaling.
Further, because of the positioning of the links, and the fact that the links do not expand or stretch when the stent is radially expanded, the overall length of the stent is substantially the same in the unexpanded and expanded configurations. In other words, the stent will not substantially shorten upon expansion.
The stent may be formed from a tube by laser cutting the pattern of cylindrical rings and undulating links in the tube. The stent also may be formed by laser cutting a flat metal sheet in the pattern of the cylindrical rings and links, and then rolling the pattern into the shape of the tubular stent and providing a longitudinal weld to form the stent.